1. Field of the Invention.
This invention relates to a photochemical process for the preparation of disilane (Si.sub.2 H.sub.6) from silane (SiH.sub.4). More particularly, it relates to the use of coherent light having a wavelength in the range from about 10.2 to about 11.2 .mu.m to induce the conversion of silane to disilane.
2. Description of the Prior Art.
Amorphous silicon films are conveniently prepared by chemical vapor deposition using either silane or disilane as a starting material. For example, amorphous silicon films can be prepared by the plasma decomposition, thermal decomposition, or photochemical decomposition of either silane or disilane. However, disilane is generally much preferred over silane as a starting material for silicon film formation. The rate of film formation obtained by the plasma or thermal decomposition of disilane is much greater than that obtained by the plasma or thermal decomposition of silane. Further, disilane is more useful than silane in photochemical processes for the preparation of silicon films since it is photochemically decomposed by light of longer wavelength which can be produced by readily available light sources such as excimer lasers and mercury lamps.
Amorphous silicon films produced by the chemical vapor deposition of disilane typically contain from about 3 to about 30 atom percent of hydrogen. Accordingly, the resulting material is often referred to as a hydrogenated amorphous silicon or an amorphous silicon-hydrogen alloy. The hydrogen results in valency saturation within the amorphous silicon, which is of importance for satisfactory electronic and photoelectric properties because free valencies can capture charge carriers within the material. This hydrogenated amorphous silicon has become an important electronic material which has found use in a variety of applications such as xerography and the fabrication of solar cells and thin-film, field-effect transistors.
A number of methods are available for the synthesis of disilane. For example, this material can be prepared by: (1) the electric discharge decomposition of silane; (2) the reduction of SiCl.sub.4 with metal hydrides such as LiAlH.sub.4 ; and (3) reaction of silicides of magnesium, aluminum, lithium, iron, and other metals with acids or their ammonium salts. Unfortunately, conventional synthetic methods typically result in either a low yield of disilane or a disilane product contaminated by impurities which are difficult to remove and which render the material unsatisfactory for use in the preparation of silicon films for electronic applications.
The SF.sub.4 -sensitized photochemical decomposition of silane has been described in detail by Longeway et al., J. Phys. Chem., Vol. 87, 354 (1983). Using unfocused radiation having a wavelength of 9.6 .mu.m from a pulsed CO.sub.2 laser, these authors studied this decomposition reaction at a fluence of 0.31 J/(cm.sup.2 pulse) over a pressure range from 3 to 16 torr, and they report that the only volatile products observed were hydrogen and disilane. They report that this finding is unusual since "all other modes of decomposition of SiH.sub.4 reported, namely pyrolytic, direct ultraviolet photolysis, direct infrared multiphoton photolysis, Hg(.sup.3 P.sub.1) photosensitized decomposition, radiolysis by CO.sup.60 .gamma.-rays, radiolysis by high-energy electrons, and electric discharge-induced decomposition result in the formation of significant amounts of Si.sub.3 H.sub.8 and higher silanes of the series Si.sub.n H.sub.2n+2."
A number of reports dealing with the unsensitized infrared photochemistry of silane have appeared in the scientific literature. For example, Basov et al., JETP Lett., Vol. 14, 165 (1971), reported that at a pressure of 228 torr, infrared radiation from a 50W continuous CO.sub.2 laser resulted in the conversion of silane to silicon and hydrogen. These authors did not, however, report the formation of disilane as a photolysis product. In addition, Deutsch, J. Chem. Phys., Vol. 70, 1187 (1979), has described the photolysis of silane at pressures below 92 torr using a pulsed CO.sub.2 laser. However, Deutsch expressly states that disilane was not observed as a photolysis product. Similarly, M. Hanabusa et al., Appl. Phys. Lett., Vol. 35, 626 (1979); and Adamova et al., Khimiya Vysokikh Energii, Vol. 11, No. 5, 347 (1977) have described the use of radiation from a CO.sub.2 laser to initiate the decomposition of silane. However, these authors fail to either suggest or disclose the formation of any silicon-containing products other than silicon itself.
The photolysis of silane by infrared radiation from a pulsed CO.sub.2 laser at a wavelength of 10.6 .mu.m over the pressure range from 10 to 22 torr and at a fluence of 1.0 J/(cm.sup.2 pulse) has been reported in detail by Longeway et al., J. Amer. Chem. Soc., Vol. 103, 6813 (1981). The observed products were H.sub.2, Si.sub.2 H.sub.6, Si.sub.3 H.sub.8, Si.sub.4 H.sub.10, Si.sub.5 H.sub.12, and a solid (SiH.sub.x).sub.n. At pressures above 14 torr, it is reported that the material balance based on volatile products falls below 80% and solid products become visible in the reaction cell. Further, it is reported that the yield of disilane decreases rapidly as the pressure increases from about 11 to about 20 torr.
Although the photochemical decomposition of silane has been extensively studied, there has been no report in the prior art of any method for the selective conversion of this material to disilane by unsensitized photolysis.